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10.2217/17460913.2.1.75 © 2007 Future Medicine Ltd ISSN 1746-0913

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Future Microbiol. (2007) 2(1), 75–84 75

How group A Streptococcus circumvents host phagocyte defensesLaura A Kwinn & Victor Nizet†

†Author for correspondenceDivision of Pediatric Pharmacology & Drug Discovery, University of California, San Diego School of Medicine, Skaggs School of Pharmacy & Pharmaceutical Sciences, 9500 Gilman Drive, Mail Code 0687,La Jolla, CA 92093-0687, USATel.: +1 858 534 7408;Fax: +1 858 534 5611;[email protected]

Keywords: complement, group A Streptococcus, innate immunity, macrophage, neutrophil, phagocytosis, Streptococcus pyogenes, virulence factor

Group A Streptococcus (GAS) is a Gram-positive bacterium associated with a variety of mucosal and invasive human infections. GAS systemic disease reflects the diverse abilities of this pathogen to avoid eradication by phagocytic defenses of the innate immune system. Here we review how GAS can avoid phagocyte engagement, inhibit complement and antibody functions required for opsonization, impair phagocytotic uptake mechanisms, promote phagocyte lysis or apoptosis, and resist specific effectors of phagocyte killing such as antimicrobial peptides and reactive oxygen species. Understanding the molecular basis of GAS phagocyte resistance may reveal novel therapeutic targets for treatment and prevention of invasive human infections.

Group A Streptococcus (GAS; Streptococcus pyo-genes) is a Gram-positive bacterium associatedwith a wide spectrum of disease conditions in thehuman host. While the majority of GAS disease islimited to superficial sites such as the pharyngealmucosa (‘strep throat’) or skin (impetigo), theorganism is also a leading agent of invasive infec-tions, including the life-threatening conditions ofnecrotizing fasciitis and toxic shock syndrome.The annual burden of invasive GAS infection isestimated at over 650,000 cases and 150,000deaths worldwide [1], reflecting the global dissem-ination of strains of enhanced virulence potential,including a highly prevalent clone of the M1T1serotype [2]. The propensity of GAS to producesystemic infection in otherwise healthy childrenand adults defines a capacity of the pathogen toresist host innate immune clearance mechanismsthat normally function to prevent microbialdissemination beyond epithelial surfaces.

Phagocytic cells such as neutrophils and macro-phages represent a critical element of innateimmunity against invasive bacterial infection. Thegeneral effectiveness of these cells in host defensebespeaks specialized functions in directed migra-tion, microbial uptake and production of a varietyof bactericidal effector molecules. In this review,we examine the multiple virulence factors of GAScapable of interfering with the host phagocytedefense system, placing a particular emphasis onrecent discoveries established through moleculargenetic analysis of the pathogen.

Impairment of phagocyte recruitmentCirculating leukocytes respond to chemotacticsignals to leave the vasculature and migrate tothe site of infection. While chemoattractantsinclude products from the bacteria cell wall

(e.g., N-formyl peptides), the strongest and mostspecific stimuli are host-derived. GAS hasevolved mechanisms to interfere with two of themost potent molecules promoting neutrophilrecruitment, the CXC chemokine interleukin(IL)-8 and the complement-derived anaphylo-toxin C5a. In this way the kinetics of the innateimmune response to GAS infection are delayed,favoring bacterial survival.

IL-8 is a multifunctional protein involved inthe migration of neutrophils out of the blood-stream and towards the site of infection. Notonly does IL-8 act a potent chemoattractant [3],it can also be found tethered to the luminal sur-face of the microvasculature where it provides astop signal to rolling neutrophils [4,5]. GAS pro-duce a protease (ScpC, also known as SpyCEP)that specifically cleaves the C terminus of IL-8,leading to functional inactivation of the chemo-kine [6]. ScpC also cleaves the murine CXCchemokines KC and MIP-2 [7]. Loss of ScpCexpression dramatically reduces GAS virulencein the mouse necrotizing fasciitis model, reflect-ing increased neutrophil influx to the site ofinfection [7].

C5a is an 11-kD fragment of the comple-ment cascade with multiple inflammatory prop-erties, including the recruitment of neutrophilsand stimulation of their bactericidal capacityagainst GAS [8]. However, GAS express anendopeptidase, ScpA, which cleaves humanC5a between His-67 and Lys-68, residueswithin the critical recognition site for leukocytesurface receptors [9]. The anchorless surfacedehydrogenase (SDH) is also shed from theGAS surface whereupon it binds and inactivateshuman C5a [10], providing another impedimentto host neutrophil chemotaxis.

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Escape from neutrophil extracellular trapsIt has recently been appreciated that, apart fromtheir phagocytic function, neutrophils can effi-ciently capture and kill microbes in the extra-cellular space. This process involves neutrophilextrusion of a matrix of DNA and histonesknown as neutrophil extracellular traps (NETs),which ensnare bacteria and subject them tomicrobicidal effectors including the granule pro-teases elastase and myeloperoxidase [11]. Withchromatin representing the principal scaffold ofNETs, the contribution of several GAS DNAseenzymes to pathogenesis has come under exami-nation. A GAS strain with mutations in threeencoded DNAses is significantly attenuated inmurine skin and systemic infection models aswell as pharyngeal infection of cynomolgusmacaques [12]. Among these was the highlyactive bacteriophage-encoded DNAse Sda1,present in the secreted proteome of the virulentM1T1 GAS clone associated with severe, inva-sive infections [13]. Sumby and collegues demon-strated significantly increased extracellularkilling of isogenic DNAse mutant GAS byhuman neutrophils using conditions known topromote NETs [12], and subsequent targetedmutagenesis and heterologous expression ofSda1 revealed that the enzyme is necessary andsufficient for promoting GAS NET degradationand resistance to neutrophil killing in vitro andin vivo [14]. Moreover, pharmacological inhibi-tion of Sda1 DNAse activity preserved hostNET function and reduced the severity of GASskin infection [14].

Interference with complement functionFollowing activation of the classical or alternativecomplement pathways, opsonization of foreignmicrobes occurs through deposition of C3b andits cleavage fragment iC3b on their surface.Complement receptors (CR) on neutrophils andmacrophages engage the bound C3b (CR1) oriC3b (CR3 and CR4) to facilitate phagocytosis.Since the complement system is capable of effi-cient self-amplification, potential host cell dam-age is mitigated by the counter-regulatoryproteins C4b-binding protein (C4BP) andfactor H (FH) that dampen the activity level ofthe classical and alternative pathways, respec-tively. GAS exhibits the capacity to acquireC4BP and FH, and these phenotypes have beenexplored for potential roles in impeding hostcomplement activation or preventing efficientopsonophagocytosis.

C4BP interferes with the assembly of themembrane-bound C3 convertase of the classicalpathway [15]. GAS is able to selectively acquirehost C4BP from human serum through theaction of the hypervariable regions of severalM-protein family members, thereby inhibitingclassical pathway activation [16,17]. Monoclonalantibody mapping studies reveal the point ofinteraction of GAS M proteins with C4BP over-laps with potential C4b binding sites [18]; how-ever, the multi-armed structure of C4BP allows itto retains its ability to act as a cofactor for C4bdegradation even when bound to M protein. Astrong correlation can be established betweenC4BP acquisition on the GAS surface and eva-sion of phagocytosis, highlighting the importanceof this innate immune resistance mechanism [19].The lack of consensus sequence motifs amongC4BP binding M-protein hypervariable regionsreflects an interesting capacity for sequence diver-gence across M types while maintaining highlyspecific ligand-binding functions [20].

FH and the variant splice form of its codinggene, FH-like protein (FHL)-1, are central fluid-phase regulators of the alternative complementpathway, functioning to accelerate the decay ofthe C3 convertase (C3bBb) and acting ascofactors for factor I-mediated degradation ofC3b [21]. M protein has long been known torestrict deposition of C3b on the GAS surface, afunction that can be correlated to resistance tophagocytosis [22]. Many GAS M proteins werefound capable of binding FH and FHL-1 pro-teins through their conserved C-repeat regionand/or hypervariable N-terminal regions [23,24].However, the overall significance of M proteinbinding to FH and FHL-1 to complement resist-ance remains a matter of debate. Affinities forFH and FHL-1 vary widely by M-type – M18possesses the highest affinity, while M1 and M3proteins, representing strains commonly associ-ated with invasive disease, show little if any bind-ing [25]. Recently, the M5 protein was shown tobind FH and FHL-1 at its N terminus, but thebacteria resisted phagocytosis equally well regard-less of the inclusion or exclusion of this N-termi-nal binding domain [26], corroborating similarobservations using an M6 serotype strain [27]. InM1 strains, the M protein is dispensible forFH/FHL-1 binding; instead, the surface-anchored protein Fba mediates binding to thesecomplement regulatory factors. Fba promotesM1 GAS survival in human whole blood andprevents deposition of C3b on the bacterial cellsurface [28].

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Deposition of complement on GAS occurs viathe classical pathway even under nonimmuneconditions, but can be blocked by the ability ofM proteins of certain GAS serotypes to bindfibrinogen, which reduces the amount of classicalpathway C3 convertase on the bacterial surface.The M-related protein Mrp, expressed by morethan half of GAS strains, also recruits fibrinogento the bacterial surface in a fashion that impairscomplement deposition [29]. Binding of fibrino-gen and fellow plasma protein albumin to theB- and C-repeats of GAS M protein plays animportant role in determining the location ofopsonic and nonopsonic epitopes [30].

The terminal complement pathway generatesthe membrane attack complex C5–9, which candisrupt bacterial cell membranes leading to hypo-osmotic lysis; although this may not be the casein Gram-positive bacteria due to the thickness ofthe peptidoglycan layer in the cell wall. M1 andM57 serotypes of GAS release the protein seruminhibitor of complement (SIC) that couples withclusterin and histidine-rich glycoprotein, twoserum regulators of membrane-attack complexactivity, consequently inhibiting complement-mediated cell lysis in an erythrocyte model [31].While preventing uptake of C567 onto the cellsurface is a measurable property of SIC [32], theprotein has several additional activities that arelikely more important contributors to GASphagocyte resistance (see below).

Interference with antibody-mediated opsonizationImmunoglobulins (Igs) generated against specificbacterial epitopes provide a second effective formof opsonization, promoting engagement anduptake by host phagocytes expressing surfacereceptors for the Ig Fc domain. GAS confoundsthis branch of host innate defense by a variety ofmeans, including molecular mimicry, antigenicdiversity, and specific proteins that degrade Ig mol-ecules, bind them in a nonopsonic fashion, orinterfere with their recognition by phagocyte Fcreceptors. In contrast to the serotype-specificpolysaccharide capsules of group B Streptococcusand Streptococcus pneumoniae, which represent pri-mary targets of protective immunity, the invariantGAS capsule consists of a homopolymer ofhyaluronic acid, identical to a major constituent ofthe mammalian extracellular matrix. This effectivemimicry provides the bacterium a protective cloaknot recognizable as a foreign antigen. And whilethe M protein on the GAS cell surface can serve asa target of protective immunity, hypervariability

of its N-terminal domain has generated morethan 100 known serovariants among which lackof cross-protection is commonplace.

The effector function of Ig may be thwartedwhen the pathogen binds its Fc region, effec-tively decorating the bacterial surface with thehost molecule in a ‘backwards’, nonopsonic ori-entation. The surface-expressed GAS fibronectin-binding protein (Sfb)I binds the Fc region of theFc region of IgG, preventing phagocytosis of IgG-coated red-blood cells (RBCs) and Ab-dependentcell cytotoxicity by macrophages [33]. Several Mprotein types and related family members alsoshow capacity for binding the Fc domains of IgGand/or IgA [34–36]. Protein H, a GAS surface pro-tein structurally related to M protein, interactswith the Fc region of IgG and inhibits IgG-dependent complement activation on the bacte-rial cell surface [37]. However, it should be notedthat there is scant evidence that Fc binding pro-teins of GAS or other bacteria can specificallybind antibacterial IgG, without first becomingsaturated with abundant nonimmune IgG.

The broad spectrum GAS cysteine proteaseSpeB has been shown to cleave IgG, -A, -M, -Dand -E antibodies in vitro [38]. This degradationoccurs even when IgG is specifically bound to abacterial antigen via its Fab regions; yet when theIgG Fc region is bound to GAS surface proteinsin a nonopsonic fashion, it is then spared fromSpeB proteolysis [39]. Mac-1/IdeS, a second GAScysteine protease [40], cleaves IgG in vitro andin vivo, targeting the lower Fc region [41,42].Mac-1/IdeS further exhibits homology to thealpha-subunit of a leukocyte B2-integrin,CD11b, which binds to FcγRIIIB (CD16) onthe surface of neutrophils inhibiting phago-cytosis and activation of the oxidative burst [43].Subsequently, the closely related Mac-2 proteinwas found to bind both Fcγ RII and III recep-tors, likewise serving to competitively inhibithost phagocyte recognition of IgG on the bacte-rial surface [41]. Like Mac-1/IdeS, Mac-2 exhibitsIgG endopeptidase activity [44]. Finally, thesecreted GAS protein EndoS hydrolyzes thechitobiose core of the asparagine-linked glycanon IgG, preventing recognition of IgG by phago-cyte Fc receptors, blocking complement activa-tion through the classical pathway and impairingopsonophagocytosis [38,45].

Avoidance of phagocytic uptakeBeyond interference with complement and anti-body-mediated opsonization, GAS employs sev-eral strategies to resist its uptake into phagocytes.

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Although the GAS hyaluronic acid capsule doesnot block C3 deposition on the bacterial surface,its antiphagocytic function is supported by sev-eral lines of evidence. Capsule-deficient GASgenerated by targeted mutagenesis of the has bio-synthetic operon or through hyaluronidase treat-ment become susceptible to phagocytic clearanceand less virulent in animal challenge models [46].Conversely, GAS variants with increased encap-sulation are generated by animal passage andmucoid strains are linked epidemiologically togreater invasive disease potential [47]. Thehyaluronic acid capsule appears to restrict accessof phagocytes to a variety of opsonins on thebacterial surface [48].

GAS also possess the capacity to utilize varioushost matrix proteins to shield their surfaceand/or promote formation of bacterial aggre-gates whose particle size may exceed the uptakecapacity of host phagocytes. For example, theSfbI protein can bind fibronectin that can inturn recruit collagen, leading to matrix deposi-tion on and between bacteria and the develop-ment of large aggregates [49]. Under physiologicalconditions, the B- and C-repeat regions of GASM protein can bind fibrinogen and albumin,thus masking them from antibody binding [30].GAS have been shown to aggregate and formintratissue microcolonies or biofilms onuncoated polystyrene surfaces or those coatedwith fibronectin or collagen [50], likely restrictingphagocyte access. Finally, the multifunctionsecreted SIC protein can colocalize with theF-actin binding domain of ezrin that links thephagocyte cytoskeleton to the plasma mem-brane. This interference can be viewed as amechanism to impair the biophysical eventsrequired or phagocytic uptake, as evidenced bythe enhanced internalization of SIC-deficientmutants by human neutrophils [51].

Cytotoxicity & phagocyte apoptosisGAS elaborate a variety of potent cytotoxins,and another important mechanism for innateimmune resistance appears to involve triggeringthe death of the phagocytic cell types before bac-terial killing can be accomplished. The pore-forming GAS β-hemolysin streptolysin S (SLS)exerts cytocidal activity on host neutrophils andthereby promotes GAS resistance to phagocytickilling [52,53]. The structurally unrelated choles-terol-binding cytolysin streptolysin O (SLO) isalso toxic to human neutrophils and impairstheir phagocytic capacity [54,55]. Consequently,both the SLS and SLO toxins are key virulence

factors in the pathogenesis of invasive GAS infec-tion [52,56,57]. The antiphagocytic functions ofSLO may be more complex than direct cytolysis, asin an epithelial cell model SLO is seen to allowGAS to avoid lysosomal localization [58]. SLO alsoserves to deliver an NADase toxin to the host cellcytoplasm in a process known as cytolysin-medi-ated translocation [59]; the NADase activitydepletes the host cell energy stores [60]. Finally,M protein released from the GAS surface can alsobe conceptualized as a toxin, forming complexeswith fibrinogen that bind to β-integrins on hostneutrophils, provoking the release of heparin bind-ing protein and inflammatory changes leading tovascular leakage and severe disease [61].

Upon phagocytosis, GAS mediate a programof accelerated neutrophil apoptosis that can becorrelated to enhanced phagocyte resistance rela-tive to a variety of other common human patho-gens [62]. Although the GAS virulence factorsinvolved in the neutrophil apoptosis differentia-tion program and their cellular targets remain tobe elucidated, epithelial cell models suggest GAScan induce a unique apoptosis pathway based oncaspase-9 release, mitochondrial dysfunction andcalcium regulation [63,64]. Additional evidencelinks GAS-induced macrophage apoptosis toactivation of matrix metalloproteases by thecysteine protease SpeB [65].

Resistance to effectors of phagocyte killingAfter phagocytic uptake of the target bacteria,neutrophils and macrophages deploy an array ofbactericidal mechanisms including vacuole acidi-fication, generation of reactive oxygen and nitro-gen species, and production of cationicmolecules including antimicrobial peptides(cathelicidins and defensins), myeloperoxidaseand lysozyme [66]. Recently it has been shownthat GAS can escape from the phagosome intothe cytoplasm of neutrophils [67]. Since viableGAS can be isolated from inside host phagocyticcells both in vitro and in vivo [68], the traditionalinterpretation of GAS as an ‘extracellular’ bacte-rial pathogen is undergoing re-evaluation. Con-sequently, increased attention has been focusedon the molecular basis of GAS survival withinphagocytes. M protein was found to inhibit thefusion of azurophilic granules with the phago-some and other membrane trafficking eventsrequired for phagosome maturation [69,70].

Lacking catalase or carotenoid pigment suchas those expressed by Staphylococcus aureus, GAShas generally been considered susceptible to host

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oxidative burst killing. However, a recent studyrevealed that GAS expression of GpoA gluta-thione peroxidase allows the organism to adaptto oxidative stress and contributes to virulencein several animal models of pyogenic GASinfection [71]. Another mechanism of neutrophilintracellular killing involves the action of cathe-licidin antimicrobial peptides, as demonstratedby the increased susceptibility of cathelicidin-deficient mice to invasive GAS infection [72].One mechanism by which GAS resists cathelici-din killing is through incorporating positivelycharged residues into its cell wall lipoteichoicacid, leading to electrostatic repulsion of thecationic antimicrobial peptide. In this fashion,D-alanylation of teichoic acids mediated by thedlt operon promotes GAS resistance to catheli-cidins and to neutrophil killing [73]. The humancathelicidin LL-37 and neutrophil α-defensinscan be bound and inactivated by GAS proteinSIC [74]. Finally, the secreted cysteine proteaseSpeB is trapped on the GAS surface bya2-macroglobulin bound to the bacterial surfaceprotein GRAB; the retained SpeB is capable ofdegrading LL-37 and protecting the bacteriaagainst its antimicrobial action [75].

Regulation of GAS virulence phenotypesExpression of GAS virulence phenotypes isunder the control of a complex set of global tran-scriptional regulators. These comprise two-com-ponent sensor kinase/response regulators, stand-alone response regulators and alternative sigmafactors. Together, these frameworks coordinateGAS response pathways, including those func-tions to subvert host phagocyte clearance. Forexample, genes positively regulated by Mga(mutli-gene regulator) include those encoding Mand M-like proteins responsible for impairingopsonization and promoting GAS neutrophilsurvival, the C5a peptidase (ScpA) targetingneutrophil chemokines, and the multifunctionalantiphagocytic SIC protein [76]. Gene expressionanalysis of phagocytosed GAS uncovered a two-component response regulator Ihk/Irr critical forpathogen survival [77]. Ihk/Irr influences expres-sion of 20% of the GAS genome, notablyincluding genes involved in cell wall formationand peptidoglycan synthesis. Elimination of theresponse regulator gene (irr) yielded a GASmutant that was unable to resist killing bycationic antimicrobial peptides and reactive oxy-gen species, and that was severely attenuated forvirulence in the mouse necrotizing skin infectionmodel [78].

The two-component GAS global regulatorCovR/S (for control of virulence) modulates theexpression of several GAS virulence factorsincluding the hyaluronic acid capsule, cysteineprotease SpeB and the cytotoxin SLS [79].CovR/S is thought to play a role in GAS adapta-tion to multiple stresses, including heat, acid andhyperosomolarity stress [80]. Recent data indicatethat genetic mutations in the covR/S locus can becorrelated with a phenotypic switch frommucosal to invasive forms of GAS infection. Incomparing the transcriptional profile of GASisolates from pharyngeal versus systemic infec-tion, two distinct patterns emerged [81]. Theswitch from the pharyngeal to the invasive pat-tern could be recapitulated by mouse passage,and was traced to mutations in either CovR orCovS that lead to increased expression ofhyaluronic acid capsule, IL-8 protease ScpC,protein SIC, DNAse Sda1, together withdecreased expression of the cysteine protease,SpeB [81]. Loss of SpeB preserved M protein andstreptokinase on the GAS surface, which allowsaccumulation and activation of the host proteaseplasmin, facilitating spread of the organism tothe deep tissues [82].

A fascinating recent study by Graham and col-legues has examined the transcriptional profile ofGAS during the course of necrotizing soft-tissueinfection and revealed a coordinated pattern ofregulatory changes promoting increased expres-sion of virulence factors involved in innateimmune resistance and host tissue damage [83].Upregulation of the Mga, IhK/Irr and Fas-BCA/X [84] regulators together with downregula-tion of negative transcription control factors suchas CovR/S and PerR [71] in sum result in therobust in vivo expression of SIC, SLS, DNAse,IL-8 protease, M protein, SodA, hyaluronic acidcapsule and other factors that can block phago-cytic uptake, promote GAS resistance to phago-cyte intracellular killing mechanisms or provelethally toxic to the phagocytes.

ConclusionsGAS derives its scientific name S. pyogenes fromthe Latin for ‘pus-generating’, consistent withneutrophilic infiltrates observed at the site ofacute GAS infection. The rising prevalence ofdeep-seated, invasive GAS infections corrobo-rates the sophisticated suite of defense mecha-nisms the pathogen has evolved to avoidclearance by the host phagocyte response. Assummarized in (Figure 1), these GAS virulencetraits interfere at multiple points, from initial

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neutrophil recruitment, to the processes ofopsonization, to bacterial entrapment anduptake, and to intracellular effectors of bacterialkilling; in several cases a single GAS moleculeimpairs multiple host defense mechanisms (e.g.,the M protein). In certain strains of GAS, forexample the globally disseminated M1T1 clone,accumulation of a larger repertoire of virulencefactors (e.g., SIC and DNAse Sda1) poses a par-ticular challenge to innate host defenses, andfinds corroboration in epidemiological associa-tions to severe disease, including necrotizing fas-ciitis and toxic shock syndrome. Moreover,mutations in the global transcriptional regulatorynetworks governing expression of GAS virulencegenes may suddenly alter the pathogen-phago-cyte equation, resulting in quantum shift towardenhanced invasive disease potential.

Future perspectiveMost bacterial pathogens associated with signifi-cant human infection also typically exist in thetransient microflora of healthy individuals in thecontext of asymptomatic colonization. Experi-mental analysis of GAS interaction with phago-cytic cells of the innate immune systemrepresents a useful paradigm for discovery andunderstanding of the underlying mechanismsdictating the development or prevention of seri-ous bacterial infection. Much work remains to bedone – of the many GAS factors reviewed in thispaper, only a subset have been definitely provento contribute to innate immune subversionin vivo, while others have been shown to pro-mote GAS phagocyte resistance in ex vivo sys-tems, and still others have simply been shown tointeract in vitro with host effector molecules in a

Figure 1. Several virulence mechanisms by which the pathogen Group A Streptococcus resists host phagocyte defenses.

C4BP: C4b-binding protein; NET: Neutrophil extracellular trap; SIC: Serum inhibitor of complement; SLS: Streptolysin S; SLO: Streptolysin O.

ChemokinedegradationScpA (C5a)ScpC (IL-8)

Sda1(DNAse)

Cloaki f opsonins

Neutrophil lysis

SLS

SLO

Breakdownof NETs

C3b

C3b

Hyaluronicacidcapsule

FibronectinFibrinogenAlbumin

M proteinM-like proteinsSfb1

Nonopsonic bindingor degradationof immunoglobulins

M proteinProtein H

Sfb1

SpeBEndoSMac1/IdsE

Interference withcomplementdeposition

C3b

Transcriptional regulation of phagocyte defense factors

M protein

Fibrinogen?? Factor H

C4BPImpairment ofphagocyteuptake

SIC

Ez

Mac-1/2

Resistance toantimicrobialpeptides

DltABCD

SIC

SpeB

CovR/S

Mga

Ihk/Irr

Others

FasBCA/X

Group A Streptococcus

FcγR

+

+

++

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fashion that could be predicted to promote thepathogen’s survival. Future investigations willbegin to define variations in host phagocytedefense dictated by genetic polymorphism orother factors such as concomitant viral infectionthat may render individual patients particularlyvulnerable. Although GAS remain universally sus-ceptible to β-lactam antibiotics including penicil-lin, considerable morbidity and mortalityassociated with invasive GAS disease reflects thepathogen’s potential for rapid tissue spread andtriggering organ system dysfunction. GAS vaccine

development has progressed slowly, complicatedby the lack of an immunogenic capsular poly-saccharide, great antigenic diversity of the surfaceM protein and caution regarding the associationof immunological cross-reactivity with the post-infectious complication of rheumatic heart dis-ease. Improved understanding of the molecularbasis of GAS phagocyte resistance may provideopportunities for therapeutic intervention,wherein novel pharmacologic agents act not to killthe bacterium directly, but rather to render itsusceptible to our normal innate defenses.

Executive summary

Human infections caused by Group A Streptococcus

• Group A Streptococcus (GAS) is a major pathogen producing a wide spectrum of superficial and invasive human infections.• Invasive GAS infections, including necrotizing fasciitis and toxic shock syndrome, strike up to 650,000 people annually with a

mortality of 10–30%.• The propensity of GAS to produce invasive infection in previously healthy individuals reflects several virulence determinants

capable of interfering with innate phagocytic defense mechanisms.

GAS factors help the pathogen avoid engagement by phagocytes

• The complement-derived chemoattractant peptide C5a is cleaved by the GAS peptidase ScpA.• The CXC chemokine interleukin-8 is degraded by the GAS serine protease ScpC.• GAS DNAses prevent capture in DNA-based neutrophil extracellular traps.

GAS molecules inhibit complement & antibody function

• The GAS hyaluronic acid capsule is nonimmunogenic, mimicking a common human matrix component, and cloaks opsonins deposited on the bacterial surface from phagocyte recognition.

• M and M-like proteins and Sfb1 recruit host matrix proteins (fibronectin, fibrinogen and collagen) to form a protective coating against opsonin recognition.

• M protein binds complement regulator C4b-binding protein to limit C3b deposition.• The protein streptococcal inhibitor of complement (SIC) interferes with formation of the C5–9 terminal complement membrane

attack complex.• M protein and Sfb1 nonopsonically bind immunoglobulin via the Fc domain.• Cysteine protease SpeB degrades immunoglobulins.

GAS impairs phagocytotic uptake mechanisms

• EndoS hydrolyses the IgG glycans involved in Fcγ receptor recognition.• Mac-1 and -2 bind to neutrophil Fc receptors, inhibiting recognition of IgG on bacterial surface.• Protein SIC impairs actin cytoskeletal arrangements required for bacterial uptake.

GAS promote phagocyte lysis and apoptosis

• The pore-forming streptolysin S and streptolysin O are cytotoxic to neutrophils and macrophages.• GAS induces an accelerated apoptosis program in human neutrophils.

GAS resists specific effectors of phagocyte killing

• D-alanylation of lipoteichoic acid, SpeB proteolysis and SIC binding interfere with host cationic antimicrobial peptide function.• GAS escape the phagosome, and M protein can block azurophilic granule:phagosome fusion.• GpoA glutathione peroxidase allows GAS to adapt to oxidative stress.

Transcriptional control of GAS phagocyte resistance phenotypes

• GAS tightly regulates the expression of these phagocyte resistance factors through an interacting web of transcriptional regulators including CovR/S, Mga and Ihk/Irr.

Future perspective

• Understanding the molecular basis of GAS phagocyte resistance may reveal novel therapeutic targets for treatment and prevention of invasive human infections.

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Affiliations• Laura A Kwinn

Division of Pediatric Pharmacology & Drug Discovery, University of California, San Diego School of Medicine, La Jolla, CA 92093, USATel.: +1 858 534 9760;Fax: +1 858 534 5611;[email protected]

• Victor NizetDivision of Pediatric Pharmacology & Drug Discovery, University of California, San Diego School of Medicine and Skaggs School of Pharmacy & Pharmaceutical Sciences, 9500 Gilman Drive, Mail Code 0687, La Jolla, CA 92093-0687, USATel.: +1 858 534 7408;Fax: +1 858 534 5611;[email protected]

how group a streptococcus circumvents host phagocyte...

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